1. Field of the Invention
The present invention relates a capacitive touchscreen, and more particularly, to pattern design for electrodes of a touchscreen.
2. Description of the Prior Art
Since touch is the most intuitive and natural way in human-machine interaction, touchscreens thus has already been widely applied in personal computers, tablets, smartphones, portable devices, handheld devices, and other fields. Recently, capacitive touch screens are developed rapidly and gradually replaces usage of touchscreens based on other technologies, such as resistive touchscreens.
A capacitive touchscreen utilizes a capacitive touch sensitive device (i.e., an active sensing array), which includes drive electrodes and senses electrodes, works with corresponding control and sense circuitry, to sense a user's touch and determine a touch location. A driving pulse is applied to the drive electrodes on a line-by-line basis. Accordingly, charges impressed on the driving electrode in the presence of the driving pulse capacitively couple to the sense electrodes of the touch sense device that are intersected. This leads to a measurable current and/or voltage on the sensing electrodes. The relationship between the driving pulse and signal measured on the sensing electrodes is related to the capacitance coupling the driving and sensing electrodes. It is therefore possible to measure capacitances (i.e., mutual-capacitance) of coupling capacitors at the surroundings according to the driving signal and the measured sensing signals. When bringing a touch object (e.g. a finger or conductive stylus) near the touch sensitive device system, it changes a local electric field between the drive electrode and the sense electrode (since the touch object is substantially connected to a ground, the electric field will be absorbed), which reduces the mutual-capacitances measured at these surroundings. Hence, a touch location of the touch object can be accurately determined with a decrease in measured mutual-capacitances.
In some operation condition, the touch sensitive device may not share a common ground with a touch object. For example, when the device using the capacitive touchscreen is placed on an electrical insulation object, such as a wooden/plastic table, a bed, a carton, a plastic box or the like, the touch sensitive device and the touch object don't share a common ground (assuming the users stands on the floor). Under such conditions, the touch object can be considered floating with respect to the ground to which the touch sensitive device is connected. Hence, the touch object becomes a floating conductor. This introduces a capacitance difference between the touch object and the touch sensitive device with respect to the ground the touch device is connected to. FIG. 1A and FIG. 1B illustrate how such conditions has an influence on the measured mutual-capacitance between the drive electrode and the sense electrode. In FIG. 1A, the touch object FINGER shares a common ground with the touch sensitive device (i.e., the sense electrode RX and the drive electrode TX). Thus, when the touch object FINGER is brought near the touch sensitive device, a measured mutual-capacitance between the sense electrode RX and the drive electrode TX decreases because the touch object absorbs the electric field therebetween. However, in FIG. 1B, the touch object FINGER does not share a common ground with the touch sensitive device. A capacitance difference C will be added between the sense electrode RX and the drive electrode TX, such that the decrease in measured mutual-capacitance become significantly smaller compared to the case where the touch object FINGER shares a common ground with the touch sensitive device. Table 1 shows the difference between two conditions.
TABLE 1Common groundNon-common groundCm (before touch)1.5 pF1.5 pFCm (after touch)1.2 pF1.4 pF
As shown by the Table 1, the non-common ground condition causes an unrecognizably tiny sense signal (the difference in measured mutual capacitance (Cm) before the touch is applied and after the touch is applied). This is difficult for processing circuitry of the touch sensitive device to analyze the sense signal to further the touch location.
There may be some approaches that can fix such problem. However, as the touchscreen could be operated in different operation conditions, such as by handheld, placed on the table, on placed on the bed, the capacitance difference between touch object and the touch sensitive device could be quite different in each operation condition. Hence, there is a need to provide an approach to make the touch sensitive device suitable to different operation conditions and ensure that the touch sensitive device can always provide a recognizable sense signal regardless of the non-common ground related capacitance difference.